Semiconductor nanowires are elongate structures having at least one dimension in the range from about 1 nm and 100 nm. Nanowires are nanoscale building blocks useable in such applications as nanoscale electronic devices and nanoscale photonic devices. Semiconductor nanowires allow such devices to be fabricated without the need for advanced lithographic techniques. The electrical and optical properties of the nanowires are strongly affected by the nanowires' diameter and cross-sectional shape. Some applications require the synthesis of nanowires with well-controlled diameters and cross-sectional shapes.
A number of different methods have been used to grow semiconductor nanowires. Some of the available methods are described by Younan Xia et al. in One-Dimensional Nanostructures: Synthesis, Characterization and Applications, 15 ADV. MATER. 353–389 (March 2003). Vapor-liquid-solid (VLS) growth is one method widely used to synthesize nanowires. A typical VLS process involves the catalytic decomposition of gaseous precursors on the surface of metal catalyst nanoparticles and the subsequent nucleation and growth of single-crystal nanowires. A number of different processes exist that employ VLS growth. These include laser ablation, chemical beam epitaxy and chemical vapor deposition (CVD).
Chemical vapor deposition processes such as metal-organic chemical vapor deposition (MOCVD) are widely used in the semiconductor industry for depositing layers of semiconductor materials. CVD reactors are commercially available, as are high-purity precursors for most single-element and compound semiconductor materials and p-type and n-type dopants for such materials. However, attempts to grow nanowires by MOCVD have typically yielded nanowires that taper significantly. Such nanowires progressively decrease in cross-sectional area along their length from base to tip. Such nanowires therefore have non-uniform cross-sectional areas and are therefore unsuitable for use in applications that need nanowires with a uniform cross-sectional area along their length.
Nanowires grown by laser ablation and chemical beam epitaxy have been reported as having substantially uniform cross-sectional areas. However, laser ablation employs equipment and processes that are not useable for other purposes, and laser ablation typically cannot be used to fabricate other parts of a device of which the nanowires would form part. Chemical beam epitaxy is not commonly used in industrial semiconductor manufacture.
What is needed, therefore, is a way of using a chemical vapor deposition process to grow nanowires that consistently have a uniform cross-sectional area along their length.